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The race to detect dark matter: Dark matter makes up most of the Universe, but no one knows what it is. Now physicists around the world are competing to detect new exotic particles that could explain all

25 April 1992

By NINA HALL and PETER F. SMITH

One of the most puzzling aspects of our Universe is that most of its mass seems to be invisible. The galaxies of stars that we see account for between one and 10 per cent of it. During the past 15 years, it has become clear that a new form of matter called ‘dark matter’ is needed to explain observations. This has brought together astronomers and particle physicists to try to discover what new objects or subatomic particles could be responsible for it. There is now a worldwide race, in which Britain is a key participant, to detect the dark matter in our Galaxy.

How do we know that dark matter exists? Our own Galaxy provides clear and dramatic evidence. The mass of an object can be calculated from its gravitational pull on another body. For example, by knowing the Moon’s speed and distance from the Earth you can work out the gravitational pull needed to keep it in orbit. From this you can calculate the weight of the Earth (6 x 10 21 tonnes). In the same way you can work out the weight of the Sun from the Earth’s orbit. But what happens if we try to use the same method to find the weight of the Galaxy?

The Galaxy consists of 100 billion stars – including our Sun – held in a cluster by gravity. It rotates about its centre, so the outer stars and gas must be held in place by the gravitational pull of the rest. From this you can calculate the total mass in the Galaxy. But …